Matt,
After you have created the custom vehicle, the process of selecting that vehicle for load generation involves the steps shown in the following figure.
Once you select the vehicle, the process for defining the remaining parameters for load generation is the same as it is for a standard vehicle available in the program database. The file named STAADBeava.CHM that is available in the folder
C:\SProV8i SS5\STAAD\Help
has a tutorial on the steps involved.